1. Field of the Invention
The present invention relates to an optical cable for telecommunication, in particular to an optical cable comprising at least one single-mode optical fiber.
2. Description of the Related Art
The use of single-mode optical fibers in fiber-to-the-premises (FTTP) applications, including fiber-to-the-home (FTTH) and fiber-to-the-building (FTTB) applications, generally require low bending loss of optical signals transmitted through the fibers, also under stringent installation constraints that may impose tight bend radii, e.g., due to sharp cornering in buildings or compression of optical fibers. In particular, cabling and hardware applications aimed to miniaturize passive field equipment, e.g., local convergence cabinets or storage boxes, and the development of multi-dwelling units (MDUs) require fiber designs with superior bending capabilities. In addition, coarse wavelength division multiplexing systems (CWDM) and passive optical network (PON) systems may also need employment of bend-insensitive optical fibers.
In order to standardize the bending performance of optical fibers particularly suited for FTTP applications, the ITU-T (International Telecommunications Union, ITU Telecommunication Sector) has recently developed recommendation G.657 that defines single-mode fibers with enhanced bending performance.
To conform to international standards, besides bending capabilities, fiber performance is evaluated also on other relevant optical parameters such as the cut-off wavelength and the mode field diameter (MFD). A parameter that can be useful for finding a compromise between the MFD, the cut-off wavelength and bending losses is the so-called MAC number, which is the ratio of the MFD and the cut-off wavelength.
It has been observed that in order to obtain low bending losses, the MAC number should be reduced.
In Characterization of the bending sensitivity of fibers by the MAC value, by C. Unger and W. Stocklein, published in Optics Communications, vol. 107 (1994), pages 361-364, macro- and micro-bending performance of matched-cladding fibers and correlation with the MAC number are investigated. The articles states that the bending behavior of step-index fibers is completely characterized by the MAC number and that macro- and micro-bending losses increase with increasing of MAC number.
WO patent application No. 01/27667 discloses a single-mode fiber operating in wavelengths between 1300 nm and 1700 nm and having reduced bending losses, the fiber comprising a MAC number not more than 7.8. Preferred fibers are said to have an MFD of 8.6 μm or less at a wavelength of 1310 nm and a cabled cut-off wavelength which is at most about 1330 nm.
US patent application No. 2007/0077016 describes an optical fiber having low bending losses in which the refractive index profile is selected to provide a MAC number of not more than 7.0, a zero dispersion wavelength of less than 1450 nm, and a 20-mm diameter bending loss at 1550 nm of not more than 5.0 dB/m. Micro-bend performance are said to be improved by a certain combination of primary and secondary coating. The primary coating has a Young's modulus of less than 1.0 MPa and a glass transition temperature of less than −25° C. and the secondary coating, which contacts and surrounds the primary coating, has a Young's modulus of greater than 1200 MPa.
Designs of optical fibers with depressed refractive-index regions, which are tailored to a reduction of bending losses, have been developed. For example, US patent application No. 2007/0280615 describes an optical fiber design usable in FTTH and FTTC (fiber-to-the-curb) transmission systems. The fiber structure comprises a central core, a first intermediate cladding, a first depressed cladding, a second intermediate cladding and a second depressed cladding. The described fiber may have MAC ratios up to about 8.2.
Y. Kitayama and S. Tanaka show in Length dependence of effective cutoff wavelength for single-mode fiber, published in Electronics and Communications in Japan, Vol. 68, No. 7, (1985), pages 104-113, that the effective cut-off wavelength of reel-wound fibers shifts to lower wavelengths along the distance. The article describes experiments of effective cut-off wavelengths measured for different reel radius performed on a 6-fiber cable where the fibers are wound around a central member with pitch of 200 mm (equivalent to a bend radius of 1084 mm) and length of 1 km.
JP patent application No. 2004198523 discloses an optical fiber module used in a Raman amplifier. In the module, the effective cut-off wavelength is shortened by winding an optical fiber for Raman amplification in a coiled form.
U.S. Pat. No. 5,590,233 concerns a cable for use in a distribution network, which comprises a plurality of optical fibers, each fiber being provided with a substantially hermetic coating and including over said hermetic coating a coating of plastics material, the cable further including an outer protective sheath of plastics material surrounding said optical fibers, wherein each of said optical fibers has a mode field diameter lying in the range of 7 μm to 9 μm at around 1550 nm, and a cutoff wavelength that is less than or equal to 1350 nm. To improve the mechanical quality of the cable, the optical fibers can be conventionally twisted along the length of the cable.
Q. Wang et al. in Theoretical and Experimental Investigations of Macro-bend Losses for standard single mode fibers, published in Optics Express Vol. 13, 13 Jun. 2005, pages 4476-4484, presents theoretical and experimental investigations of macro-bend losses for standard fibers SMF28 showing that the inner primary coating layer has an impact on the bending losses.
U.S. Pat. No. 6,477,297 describes a method for assembling a plurality of optical fibers for forming fiber pigtailed component aimed to a reduction of the optical impact on macrobending. Although the nominal cut-off wavelength of SMF-28 used for pigtails is approximately 1280 nm, for a batch of fibers, the actual cut-off wavelength is distributed across a Gaussian distribution. The disclosed method comprises the steps of: selecting, from a plurality of optical fibers characterized by a common nominal cut-off wavelength and an actual cut-off wavelength such that the actual cut-off wavelength of each one of this plurality of fibers is the same as the nominal cut-off wavelength or differs slightly from the nominal cut-off wavelength due to manufacturing tolerances; only fibers with actual cut-off wavelength larger than λmin, where λmin is a predetermined minimum acceptable cut-off wavelength of the selected fibers; and bending at least one section of at least one of these selected fibers such that this bent section has a bend radius R, where 12 mm<R<18 mm.